1. Field of the Invention
The present invention relates to a system and method of testing of a conventional telephone line and, more particularly, to a system and method determining the loop insertion loss of a telephone line single-endedly.
2. Description of Related Art
One of the most useful predictors of the maximum data rate achievable by a telephone line broadband service, such as DSL, is the electrical property of a so-called 300 kHz loop insertion loss. “Loop” is an industry term synonymous with “line”. Conventional measurement of loop insertion loss requires a transmitter to be placed at one end of the line and a receiver at the other, thus making it a double-ended test. Double-ended tests are slow and costly for telephone companies because they require a “truck roll” to the customer end of each line to be tested. Far superior—and difficult—would be to measure insertion loss single-endedly from a central office location where all of the lines in an area originate.
It would also be desirable for any central office-based test apparatus to be able to make use of the so-called Numbered Test Trunk (NTT) provisions of the office's PSTN switch(es). The NTT interface enables convenient one-at-a-time test access to all telephone lines served by a switch. However, this type of test access introduces a significant technological hurdle. The “switch fabric” (a term describing the electrical makeup of the test path through the switch) has a narrowband frequency response, in that it begins attenuating signals above approximately 10 kHz in frequency. The attenuation becomes extremely severe well before 300 kHz. Thus, in addition to measuring 300 kHz insertion loss single-endedly, any successful technique must also be able to do so without the use of actual 300 kHz signals.
Another hurdle exists when testing through the switch fabric (most notably with the Lucent 5ESS, the prevalent switch type in the U.S.). Non-uniform frequency responses are often exhibited, both between different lines, and between different accesses to the same line. If left unaccounted for, these differences in frequency response would have a profound effect on measured insertion loss, rendering the measurement so erroneous or inconsistent as to have little or no value.
Described herein is a novel technique that solves these problems and others and successfully and single-endedly measures insertion loss through a PSTN switch (or other test path).